1. Field of the Invention
This invention relates to a waveform shaping circuit which is so designed as to intermittently operate and adapted for use with a mobile communication apparatus, and a receiver using such a waveform shaping circuit. More particularly, this invention is directed to such an arrangement with an improved transient characteristic which occurs when an input signal having digital data mingled therewith is subjected to waveform-shaping.
2. Description of the Prior Art
To have a better understanding of the present invention, description will first be made of the prior art with reference to FIGS. 1 and 2 of the accompanying drawings. FIG. 1 is a circuit diagram showing an example of conventional waveform shaping circuit adapted for use with a paging device or the like, wherein reference numeral 2 indicates an input terminal to which is applied an input signal such as a digital data signal having an arbitrary waveform or a digital data signal retaining a relatively square waveform, and reference numeral 3 indicates an output terminal at which is obtained a waveform-shaped digital data signal. Resistors 25 and 26 are connected at one end to the input terminal 2 and also coupled at the opposite end to inverting input terminal and non-inverting input terminal of a differential amplifier 1 which has a feedback resistor 28 connected across the inverting input terminal and output terminal 3 thereof so as to constitute a feedback circuit. A capacitor 16 is coupled to the connection point between the resistor 25 and the non-inverting input terminal of the differential amplifier 1, and the collector of a transistor 29 is also connected thereto. A reference voltage source V.sub.R is connected to a terminal 20 which comprises the emitter of the transistor 29; and a terminal 13 is connected to the base of the transistor 29, and a control signal for boosting charge is applied thereto.
The power source voltage is intermittently applied to a waveform shaping circuit, and the control signal for boosting charge is applied to the terminal 13 in synchronism with turning-on of the power source so that a charging current i.sub.c is supplied to the capacitor 16 through the transistor 19. Input signal is supplied to the non-inverting input terminal of the differential amplifier 1 through the resistor 26. The resistor 25 and capacitor 16 constitute an integrating circuit 27 by which the input signal is smoothed. Such smoothed input signal is passed to the differential amplifier via the non-inverting input terminal so that the input signal is subjected to waveform-shaping.
In the waveform shaping circuit of FIG. 1, input signal is supplied to the input terminal 2. The waveform of the input signal is shown at (a) in FIG. 2(II). When the control signal for boosting charge is applied to the control terminal 13 as shown in FIG. 2(I), the transistor 29 is rendered operative so that the charging current i.sub.c is caused to rapidly flow in the capacitor 16 and thus charging voltage at the capacitor 16 builds up as shown by a charging curve (b). The charging voltage increases up to a value close to a voltage (V.sub.R -V.sub.CE(SAT)) which is equal to the reference voltage V.sub.R minus the saturation voltage V.sub.CE(SAT) of the transistor 29. When the control signal is interrupted, the voltage charged at the capacitor 16 drops down to a voltage level (d) corresponding to the smoothed input signal superimposed upon DC bias voltage, as indicated by a discharge curve (c) (see FIG. 2(II)).
As will be seen from FIG. 2, it is during the time period t.sub.1 to t.sub.2 that data error tends to occur, and this discharge time period is proportional to the time constant of the integrating circuit 27. With this type of waveform shaping circuit, digital data signal with a constant duty ratio is more stably obtained as the time constant of the integrating circuit 27 is increased. Disadvantageously, however, as the time constant of the integrating circuit 27 increased, the time period (t.sub.1 to t.sub.2) during which data error tends to occur, i.e., the time period during which the reference voltage level occurring after the boosting charge transiently becomes astable, becomes longer.